1. Field of the Invention
The present invention relates in general to the field of electronics, and more specifically to a system and method for mapping an output of a lighting dimmer in a lighting system to predetermined lighting output functions.
2. Description of the Related Art
Commercially practical incandescent light bulbs have been available for over 100 years. However, other light sources show promise as commercially viable alternatives to the incandescent light bulb. Gas discharge light sources, such as fluorescent, mercury vapor, low pressure sodium, and high pressure sodium lights and electroluminescent light sources, such as a light emitting diode (LED), represent two categories of light source alternatives to incandescent lights. LEDs are becoming particularly attractive as main stream light sources in part because of energy savings through high efficiency light output and environmental incentives such as the reduction of mercury.
Incandescent lights generate light by passing current through a filament located within a vacuum chamber. The current causes the filament to heat and produce light. The filament produces more heat as more current passes through the filament. For a clear vacuum chamber, the temperature of the filament determines the color of the light. A lower temperature results in yellowish tinted light and a high temperature results in a bluer, whiter light.
Gas discharge lamps include a housing that encloses gas. The housing is terminated by two electrodes. The electrodes are charged to create a voltage difference between the electrodes. The charged electrodes heat and cause the enclosed gas to ionize. The ionized gas produces light. Fluorescent lights contain mercury vapor that produces ultraviolet light. The housing interior of the fluorescent lights include a phosphor coating to convert the ultraviolet light into visible light.
LEDs are semiconductor devices and are driven by direct current. The lumen output intensity (i.e. brightness) of the LED varies approximately in direct proportion to the current flowing through the LED. Thus, increasing current supplied to an LED increases the intensity of the LED, and decreasing current supplied to the LED dims the LED. Current can be modified by either directly reducing the direct current level to the white LEDs or by reducing the average current through pulse width modulation.
Dimming a light source saves energy when operating a light source and also allows a user to adjust the intensity of the light source to a desired level. Many facilities, such as homes and buildings, include light source dimming circuits (referred to herein as a “dimmer”).
FIG. 1A depicts a lighting circuit 100 with a conventional dimmer 102 for dimming incandescent light source 104 in response to inputs to variable resistor 106. The dimmer 102, light source 104, and voltage source 108 are connected in series. Voltage source 108 supplies alternating current at line voltage Vline. The line voltage Vline can vary depending upon geographic location. The line voltage Vline is typically 110-120 Vac or 220-240 Vac with a typical frequency of 60 Hz or 70 Hz. Instead of diverting energy from the light source 104 into a resistor, dimmer 102 switches the light source 104 off and on many times every second to reduce the total amount of energy provided to light source 104. A user can select the resistance of variable resistor 106 and, thus, adjust the charge time of capacitor 110. A second, fixed resistor 112 provides a minimum resistance when the variable resistor 106 is set to 0 ohms. When capacitor 110 charges to a voltage greater than a trigger voltage of diac 114, the diac 114 conducts and the gate of triac 116 charges. The resulting voltage at the gate of triac 116 and across bias resistor 118 causes the triac 116 to conduct. When the current I passes through zero, the triac 116 becomes nonconductive, (i.e. turns ‘off’). When the triac 116 is nonconductive, dimmer output voltage VDIM is 0 V. When triac 116 conducts, the dimmer output voltage VDIM equals the line voltage Vline. The charge time of capacitor 110 required to charge capacitor 110 to a voltage sufficient to trigger diac 114 depends upon the value of current I. The value of current I depends upon the resistance of variable resistor 106 and resistor 112.
In at least one embodiment, the duty cycles, and, correspondingly, the phase angle, of dimmer output voltage VDIM represent dimming levels of dimmer 102. The limitations upon conventional dimmer 102 prevent duty cycles of 100% to 0% and generally can range from 95% to 10%. Thus, adjusting the resistance of variable resistor 106 adjusts the phase angle and, thus, the dimming level represented by the dimmer output voltage VDIM. Adjusting the phase angle of dimmer output voltage VDIM modifies the average power to light source 104, which adjusts the intensity of light source 104.
FIG. 1B depicts a lighting circuit 140 with a 3-wire conventional dimmer 150 for dimming incandescent light source 104. The conventional dimmer 150 can be microcontroller based. A pair of the wires carries the AC line voltage Vline to light source controller/driver 152. In another embodiment, the line voltage Vline is applied directly to the light source controller/driver 152. A third wire carries a dimmer output signal value DV to light source controller/driver 152. In at least one embodiment, the dimmer 150 is a digital dimmer that receives a dimmer level user input from a user via, for example, push buttons, other switch types, or a remote control, and converts the dimmer level user input into the dimmer output signal value DV. In at least one embodiment, the dimmer output signal value DV is digital data representing the selected dimming level or other dimmer function. The dimmer output signal value DV serves as a control signal for light source controller/driver 152. The light source controller/driver 152 receives the dimmer output signal value DV and provides a drive current to light source 104 that dims light source 104 to a dimming level indicated by dimmer output signal value DV.
FIG. 2 depicts the duty cycles and corresponding phase angles of the modified dimmer output voltage VDIM waveform of dimmer 102. The dimmer output voltage oscillates during each period from a positive voltage to a negative voltage. (The positive and negative voltages are characterized with respect to a reference direct current (dc) voltage level, such as a neutral or common voltage reference.) The period of each full cycle 202.0 through 202.N is the same frequency as Vline, where N is an integer. The dimmer 102 chops the voltage half cycles 204.0 through 204.N and 206.0 through 206.N to alter the duty cycle and phase angle of each half cycle. The phase angles are measurements of the points in the cycles of dimmer output voltage VDIM at which chopping occurs. The dimmer 102 chops the positive half cycle 204.0 at time t1 so that half cycle 204.0 is 0 V from time t0 through time t1 and has a positive voltage from time t1 to time t2. The light source 104 is, thus, turned ‘off’ from times t0 through t1 and turned ‘on’ from times t1 through t2. Dimmer 102 chops the positive half cycle 206.0 with the same timing as the negative half cycle 204.0. So, the phase angles of each half cycle of cycle 202.0 are the same. Thus, the full phase angle of dimmer 102 is directly related to the duty cycle for cycle 202.0. Equation [1] sets forth the duty cycle for cycle 202.0 is:
                              Duty          ⁢                                          ⁢          Cycle                =                                            (                                                t                  2                                -                                  t                  1                                            )                                      (                                                t                  2                                -                                  t                  0                                            )                                .                                    [        1        ]            
When the resistance of variable resistance 106 is increased, the duty cycles and phase angles of dimmer 102 also decreases. Between time t2 and time t3, the resistance of variable resistance 106 is increased, and, thus, dimmer 102 chops the full cycle 202.N at later times in the positive half cycle 204.N and the negative half cycle 206.N of full cycle 202.N with respect to cycle 202.0. Dimmer 102 continues to chop the positive half cycle 204.N with the same timing as the negative half cycle 206.N. So, the duty cycles and phase angles of each half cycle of cycle 202.N are the same.
Since times (t5-t4)<(t2-t1), less average power is delivered to light source 104 by the sine wave 202.N of dimmer voltage VDIM, and the intensity of light source 104 decreases at time t3 relative to the intensity at time t2.
FIG. 3 depicts a measured light versus perceived light graph 300 representing typical percentages of measured light versus perceived light during dimming. The multiple dimming levels of dimmer 102 vary the measured light output of incandescent light source 104 in relation to the resistance of variable resistor 106. Thus, the measured light generated by the light source 104 is a function of the dimmer output voltage VDIM. One hundred percent measured light represents the maximum, rated lumen output of the light source 104, and zero percent measured light represents no light output.
A human eye responds to decreases in the measured light percentage by automatically enlarging the pupil to allow more light to enter the eye. Allowing more light to enter the eye results in the perception that the light is actually brighter. Thus, the light perceived by the human is always greater than the measured light. For example, the curve 302 indicates that at 1% measured light, the perceived light is 10%. In one embodiment, measured light and perceived light percentages do not completely converge until measured light is approximately 100%.
Many lighting applications, such as architectural dimming, higher performance dimming, and energy management dimming, involve measured light varying from 1% to 10%. Because of the non-linear relationship between measured light and perceived light, dimmer 102 has very little dimming level range and can be very sensitive at low measured output light levels. Thus, the ability of dimmers to provide precision control at low measured light levels is very limited.